Turbomachinery applications, such as but not limited to centrifugal compressors (hereinafter a centrifugal compressor is used as a primary example), require shaft sealing in order to prevent leakage of process gas into the atmosphere, or migration of process gas across the seals or into the bearing side of the equipment. A variety of shaft sealing mechanisms exist, including but not limited to, labyrinth seals (a radial seal), oil film seal rings (a radial seal), mechanical contact seals (a face seal), and the most elaborate type of shaft seal—the dry gas seal (a face seal). Dry gas seals provide the best barrier to leakage of all seal types, and are therefore considered the incumbent prior art technology for comparison to the subject invention. Dry gas seals utilize rotating rings that contain micron-sized grooves that allow for face “lift-off” during operation, and thus allow for a minimal controlled amount of seal leakage.
State-of-the-art solutions for dry gas seals include single seals, tandem seals, and double-opposed seals.
In the prior art, tandem dry gas seal arrangement include a primary seal is comprised of a primary (stationary) ring and a mating (rotating) ring, which withstands total pressure, and the secondary seal is comprised of a stationary ring and mating (rotating) ring, which acts as a backup. Primary rings are typically made from carbon, and mating rings are typically made using tungsten carbide, silicon carbide or silicon nitride. Dry sealing gas (which is typically the same as the process gas, but treated) is injected, and is typically at least 50 psi above the process side pressure. Most of the injected dry sealing gas flows across an inner labyrinth seal and into the compressor (into the process gas). A small amount of the dry sealing gas flows across the primary seal and then out of a primary vent. For a single seal arrangement, there is only a primary face and a mating face, with no secondary seal. Otherwise the functionality of a single seal is similar to that of a tandem seal, except for the fact that leakage across the primary seal gets vented, and there is no back-up seal.
Also in the current art, a “double opposed” dry gas seal arrangement is comprised of a primary inboard stationary ring and a primary inboard mating (rotating) ring. Also, there is a secondary outboard stationary ring and a secondary outboard mating (rotating) ring. In this case, an inert gas (such as nitrogen) is injected and flows past both seal faces. The seal gas supply flowing past the primary (inboard) seal migrates into the process side flow, and the seal gas supply flowing past the secondary (outboard) seal goes to a vent.
For the current art types of seals (single, tandem or double-opposed), tandem seals are the most widely-accepted arrangement for industry when considering the best sealing possibility.
There are many drawbacks to any of the above current art seal types. These drawbacks include:
Process gas leakage, although attempted to be controlled, may still exit the primary vent, and must be flared.
Leakage of inert gas into the process side (for a double-opposed seal) can cause problems inside the compressor by affecting the process.
Double-opposed dry gas seals have not been widely accepted in the industry (tandem seals are more widely accepted).
Flow across the seal faces for dry gas seals can contain sealing gas and process gas. This is a bad fundamental design to allow these gases across the seal gap.
Dry gas seals have a large flow of seal gas past the inner labyrinth seal, into the process side of compressors.
Dry gas seals have very high flow rates.
Seal faces can “ring” together during shut-down, resulting in very high start-up torques, or no-start conditions.
Seal faces can distort as a result of high pressures or local heating.
Small gaps result in heat generation at high speeds.
Fluids will carbonize from shear in small gaps.
A high percentage of seal failures are at start-up or shut-down.
Seal failures can result from impure seal gas supply.
While any seal may be optimized for a specific application to reduce some of these drawbacks, an improved seal that may minimize some or most of the drawbacks is desirable.
Current arrangements of typical rotating assemblies in turbomachinery, such as centrifugal compressors, include two radial bearings, two shaft seals (commonly dry gas seals), a thrust bearing, and likely a balance piston (to minimize thrust loading). Furthermore, each of these components has accompanying inputs and outputs. For example, dry gas seals have incoming buffer gas, incoming inert seal gas, incoming separation gas, and two outgoing vents. Radial bearings have oil inputs and drains, along with a bearing box vent. Also, thrust bearings have an oil input and output. Hence, a typical arrangement will have six vents, two drains, three seal gases, and oil.